(b. Aug. 9, 1896, Neuchâtel, Switz.--d. Sept. 17, 1980, Geneva), Swiss psychologist who was the first to make a systematic study of the acquisition of understanding in children. He is thought by many to have been the major figure in 20th-century developmental psychology.
Piaget's early interests were in zoology; at the age of 10 he published an article on his observations of an albino sparrow, and by 15 his several publications on mollusks had gained him a reputation among European zoologists. At the University of Neuchâtel, he studied zoology and philosophy, receiving his doctorate in the former in 1918. Soon afterward, however, he became interested in psychology, combining his biological training with his interest in epistemology. He first went to Zürich, where he studied under Carl Gustav Jung and Eugen Bleuler, and then began two years of study at the Sorbonne in Paris in 1919.
In Paris Piaget devised and administered reading tests to schoolchildren and became interested in the types of errors they made, leading him to explore the reasoning process in these young children. By 1921 he had begun to publish his findings; the same year brought him back to Switzerland, where he was appointed director of the Institut J.J. Rousseau in Geneva. In 1926-29 he was professor of philosophy at the University of Neuchâtel, and in 1929 he joined the faculty of the University of Geneva as professor of child psychology, remaining there until his death. In 1955 he established the International Centre of Genetic Epistemology at Geneva and became its director. In more than 50 books and monographs over his long career, Piaget continued to develop the theme he first discovered in Paris, that the mind of the child evolves through a series of set stages to adulthood.
Piaget saw the child as constantly creating and recreating his own model of reality, achieving mental growth by integrating simpler concepts into higher level concepts at each stage. He argued for a "genetic epistemology," a timetable established by nature for the development of the child's ability to think, and he traced four stages in that development. He described the child during the first two years of life as being in a sensorimotor stage, chiefly concerned with mastering his own innate physical reflexes and extending them into pleasurable or interesting actions. During the same period, the child first becomes aware of himself as a separate physical entity and then realizes that the objects around him also have a separate and permanent existence. In the second, or preoperational, stage, roughly from age two to age six or seven, the child learns to manipulate his environment symbolically through inner representations, or thoughts, about the external world. During this stage, he learns to represent objects by words and to manipulate the words mentally, just as he earlier manipulated the physical objects themselves. In the third, or concrete operational, stage, from age 7 to age 11 or 12, occurs the beginning of logic in the child's thought processes and the beginning of the classification of objects by their similarities and differences. During this period, the child also begins to grasp concepts of time and number. The fourth stage, the period of formal operations, begins at age 12 and extends into adulthood. It is characterized by an orderliness of thinking and a mastery of logical thought, allowing a more flexible kind of mental experimentation. The child learns in this final stage to manipulate abstract ideas, make hypotheses, and see the implications of his own thinking and that of others.
Piaget's concept of these developmental stages caused a reevaluation of older ideas of the child, of learning, and of education. If the development of certain thought processes was on a genetically determined timetable, simple reinforcement was not sufficient to teach concepts; the child's mental development would have to be at the proper stage to assimilate those concepts. Thus, the teacher became not a transmitter of knowledge but a guide to the child's own discovery of the world.
Piaget reached his conclusions about child development through his observations of and conversations with his own children, as well as others. He asked them ingenious and revealing questions about simple problems he had devised, and then he formed a picture of their way of viewing the world by analyzing their mistaken responses.
Among Piaget's major works available in English are Le langage et la pensée chez l'enfant (1923; The Language and Thought of the Child), Le Jugement et la raisonnement chez l'enfant (1924; Judgement and Reasoning in the Child), and La Naissance de l'intelligence chez l'enfant (1948; The Origins of Intelligence in Children). He also wrote a series of books dealing separately with children's conceptions of time, space, physical causality, movement and speed, and of the world in general.

Piaget's theory :

Jean Piaget took the intellectual functioning of adults as the central phenomenon to be explained and wanted to know how an adult acquired the ability to think logically and to draw valid conclusions about the world from evidence. Piaget's theory rests on the fundamental notion that the child develops through stages until he arrives at a stage of thinking that resembles that of an adult. The four stages given by Piaget are (1) the sensorimotor stage from birth to 2 years, (2) the preoperational stage from 2 to 7 years, (3) the concrete-operational stage from 7 to 12 years, and (4) the stage of formal operations that characterizes the adolescent and the adult. One of Piaget's fundamental assumptions is that early intellectual growth arises primarily out of the child's interactions with objects in the environment. For example, Piaget believed that as a two-year-old child repeatedly builds and knocks down a tower of blocks, he is learning that the arrangement of objects in the world can be reversed. According to Piaget, children organize and adapt their experiences with objects into increasingly sophisticated cognitive models that enable them to deal with future situations in more effective ways. The older child, for instance, who has learned the concept of reversibility, will be able to execute an intelligent and logical search for a missing object, retracing his steps, for example, in order to determine where he may have dropped a set of keys. As children pass through successive stages of cognitive development, their knowledge of the world assumes different forms, with each stage building on the models and concepts acquired in the preceding stage. Adolescents in the final developmental stage, that of formal operations, are able to think in a rational and systematic manner about hypothetical problems that are not necessarily in accord with their experience. Piaget's theory is treated in greater detail below in the sections on cognitive development in infancy and childhood.